1. Field of the Invention
The invention relates generally to the fields of marine seismic data acquisition and data processing. More particularly the invention relates to methods for designing and actuating marine seismic sources, and for processing such data, in which the up-going and down-going wave-field emitted by the source can be extracted and added constructively.
2. Background Art
In seismic exploration, seismic data are acquired by imparting acoustic energy into the Earth near its surface, and detecting acoustic energy that is reflected from boundaries between different layers of subsurface rock formations. Acoustic energy is reflected when there is a difference in acoustic impedance between adjacent layers to a boundary. Signals representing the detected acoustic energy are interpreted to infer structures and composition of the subsurface rock formation structures.
In marine seismic exploration, a seismic energy source, such as an air gun, or air gun array, is typically used to impart the acoustic energy into the formations below the bottom of the water. The air gun or array is actuated at a selected depth in the water, typically while the air gun or array is towed by a vessel. The same or a different vessel tows one or more seismic sensor cables, called “streamers”, in the water. Generally the streamer extends behind the vessel along the direction in which the streamer is towed. Typically, a streamer includes a plurality of hydrophones disposed on the cable at spaced apart, known positions along the cable. Hydrophones, as is known in the art, are sensors that generate an optical or electrical signal corresponding to the pressure of the water or the time gradient (dp/dt) of pressure in the water. The vessel that tows the one or more streamers typically includes recording equipment to make a record, indexed with respect to time, of the signals generated by the hydrophones in response to the detected acoustic energy. The record of signals is processed, as previously explained, to infer structures of and compositions of the earth formations below the locations at which the seismic survey is performed.
Marine seismic data include an effect that limits the accuracy of inferring the structure and composition of the subsurface rock formations. This effect, known as source ghosting, arises because water has a substantially different density and propagation velocity of pressure waves than the air above the water surface. Source ghosting can be understood as follows. When the air gun or air gun array is actuated, acoustic energy radiates generally outwardly from the air gun or array. Half of the energy travels downwardly where it passes through the water bottom and into the subsurface rock formations. The other half of the acoustic energy travels upwardly from the gun or array and most of this energy reflects from the water surface whereupon it travels downwardly. The reflected acoustic energy will be delayed in time and also be shifted in phase by about 180 degrees from the directly downward propagating acoustic energy. The surface-reflected, downwardly traveling acoustic energy is commonly known as a “ghost” signal. The ghost signal interferes with the directly downward propagating wave-field causing constructive interference in some parts of the frequency band and destructive interference in other parts of the frequency band. This causes a sequence of notches in the spectrum, equally spaced in frequency including a notch at zero frequency (0 Hz). The frequencies of these notches in the detected acoustic signal are related to the depth at which the air gun or gun array is disposed, as is well known in the art. The effect of the source ghosting is typically referred to as the “source ghost.”
The seismic energy emitted by the source is attenuated with propagation distance because of geometrical spreading, transmission loss, and absorption. The absorption of higher-frequency energy at a greater rate than lower-frequency energy is well known in the art. Therefore, for deep penetration it is a desire to maximize the energy emitted by the source at lower frequencies. Since the source ghost has a notch at 0 Hz, it is limiting the energy in the low-frequency end. This may be improved by towing the sources at a greater depth. However, this causes the ghost notches in the spectrum to occur at lower frequencies, and hence limits the high frequency parts of the spectrum needed for high resolution imaging of shallower targets. Also, when using air gun(s) as a seismic energy source, the fundamental frequency of the gun(s) increases with increasing depth. Hence, the increase in energy in the low frequency end when towing the air-guns deeper due to the source ghost, is counteracted by the increase in fundamental frequency of the air-gun(s).
A traditional way of increasing the signal level emitted by the source across the bandwidth when using air-gun(s) is to increase the total volume of air released by the air-gun(s) and/or to increase the operating pressure. However, the maximum volume of air that can be released for every shot and the maximum air pressure is limited by the available source equipment and air-supply system. To change this can be very expensive and time consuming. Also, increasing the source strength may have an impact on marine life. Therefore, maximizing the use of the signal emitted by the source may be of great value and reduce the need to increase the energy level emitted by the source. By extracting the upward (ghosted) and the directly downward propagating wave-fields from the source, the effects of the source ghost are eliminated and the signal around all ghost notches is boosted including the notch at 0 Hz. These separated wave-fields can also be time shifted to the sea-surface or a common reference depth using the known source depth(s), then by applying a 180 phase shift to the ghosted signal, they can be summed together constructively. In this way almost all energy emitted by the source is utilized, which consequentially almost doubles the primary energy level for a given energy source.
A technique known in the art for extracting the source ghost is described in M. Egan et al., Full deghosting of OBC data with over/under source acquisition, 2007 Annual Meeting, San Antonio, Tex., Society of Exploration Geophysicists. The technique described in the Egan et al. publication includes towing a first seismic energy source at a first depth in the water, and towing a second seismic energy source at a second depth in the water. The sources are air guns or arrays thereof. The second source is also towed at a selected distance behind the first source. The first source is actuated and seismic signals are recorded corresponding to actuations of the first source. After the towing vessel has moved so that the second source is disposed at substantially the same geodetic position as the first source was at the time of its actuation, the second source is actuated and seismic signals are again recorded. A “deghosted” seismic data set is obtained using the technique described more fully in the Egan et al. publication.
One of the main issues with the over/under source technique described in the Egan et al. publication referred to above is that the number of shot positions is half compared to conventional source actuation techniques causing the fold coverage to be half. Another issue with this technique, if the seismic receivers are towed behind a vessel and hence moving from shot to shot, is that the receivers have moved a considerable distance between when the sources at different depths are actuated. To maintain the number of shot positions and fold coverage as in conventional marine seismic acquisition, and to minimize the difference in receiver positions when the sources at different depths are actuated, it is desirable to have a method for extracting the source ghost that allows sources towed at different depths to be actuated during the recording of each shot record.
A technique known in the art for actuating multiple sources during the recording of each shot record is described in U.S. Pat. No. 6,882,938 issued to S. Vaage and commonly owned with the present invention. In the described technique, multiple sources are actuated with selected variable time delays relative to the start of the seismic recording. The wave-fields emitted by each individual source can be extracted by using the coherency of the signals from one source in certain domains after correcting for the known time delays of actuating that source.